The present invention relates to the field of aerial cables and more particularly concerns a method and apparatus for breaking and removing ice accretions formed on such cables.
In cold regions, icing storms cause sleet, glaze, and rime to accumulate on exposed aerial cables and conductors. The weight of such ice deposits adds to the wind charge and increases the mechanical tensions in the cable, which may cause the cable to sag excessively or break. These problems occur with all types of cables, such as ground wires and conductors used on power and distribution line networks.
A few methods have been developed and used by electrical utility companies to counter and prevent the formation of ice on power line cables. A summary of such techniques can be found in Laforte, Allaire and Laflamme, xe2x80x9cState-of-the-art on power line de-icingxe2x80x9d, Atmospheric research, vol. 46, pp 143-158 (1998). Conventional practices for avoiding or limiting ice formation generally consist of using Joule heating by injection of high, short-circuit currents, or mechanical shocks exerted on the ice accretions.
Methods using high, short-circuit currents consist of heating cables sufficiently to prevent heavy ice formation and cause adhering ice deposits to fall off. In the case of electrical conductors, the heating is done by injecting therein high intensity currents supplied by means of external transformers or normal transformers in special connections (see for example U.S. Pat. Nos. 4,085,338; 4,119,866; and 4,135,331, all to GENRICKH et al.). In the case of ground wires, which are non-energised, de-icing by Joule heating requires the use of an external power source supplying the high current needed. Such a practice has certain inherent disadvantages, one of them being the heavy current load required to raise the temperature sufficiently to cause the ice deposit to shed. Often, the capacity of the transmission system supplying the current is not sufficient to accomplish this task, and excessively large and expensive auxiliary transformers and rectifiers are required.
Mechanical methods include various miscellaneous devices, such as for example shown in Polhman and Landers, xe2x80x9cPresent state-of-the-art of transmission line icingxe2x80x9d, IEEE Transactions on power Apparatus and Systems, vol. PAS-101, no. 8, pp. 2443-2450 (1982). Most of them are generally used for ad-hoc de-icing operations, and need to be brought to the site after the icing events. The most common devices are various hand de-icing tools, which are used by linemen who access the iced cables by means of man-lifts or helicopters. Other methods, such as rollers (see Manitoba Hydro, xe2x80x9cAtelier sur la gestion des tempxc3xaates de verglasxe2x80x9d, Association Canadienne de l""Électricitxc3xa9 (ACE), February 1993), mobile robots (see Montambault et al., xe2x80x9cPreliminary Results on the Development of a Teleoperated compact Trolley for Live-in Workingxe2x80x9d TransÉnergie, 7p. 2000) and explosive devices (see Canadian patent application no. 2,281,740 (GAGNON)), which are under development, are remotely operated by people on the ground. However, they have to be installed on the iced cables prior to de-icing. All these ad-hoc de-icing operations have to be done according to very strict procedures to ensure public and worker safety. Electro-expulsive sheaths (U.S. Pat. No. 6,207,939 (ALLAIRE)), automated robots moving along cables (Montambault et al., xe2x80x9cPreliminary Results on the Development of a Teleoperated compact Trolley for Live-in Workingxe2x80x9d TransÉnergie, 7p. 2000), and vibrating devices at the centre of the energised conductors (Hansen and Wahl, xe2x80x9cDe-icing of Power Linesxe2x80x9d Protura As, 21p. 2000), are permanently installed. That is why they are subjected to long exposures, to severe environmental conditions between icing events, and require regular maintenance and refurbishment.
All mechanical devices and methods developed for de-icing conductors and ground wires of electrical lines have the inconvenience of being very expensive with regards to the man-hours required and the need to shut down the line during the de-icing operation, thus interrupting service to clients. Besides, most of the mechanical devices developed up to now, except for explosive and vibrating devices, involve an external force acting directly on the ice accretion, instead of acting on the cable itself. Explosive and vibrating devices on the other hand do apply mechanical forces directly onto the cable where they are placed. Explosive devices are fixed at one extremity of the cable and ice is removed by means of a few high intensity shock waves. Mechanical vibrators are installed at the centre of the cable which is being vibrated for de-icing. Both of these types of devices however have many disadvantages. The explosive devices require to be brought and installed at the level of aerial cables, which are de-iced after icing storms, span by span. They also require to be regularly reloaded with new explosive materials after a limited number of shocks. Moreover, repeated high intensity shocks may damage cable at their attachments. The removing of ice accretions on a span while other spans remain iced can produce unbalanced loads, damaging cable at their attachments. In the case of mechanical vibrators at cable centre, they require to be powered using electrical wires disposed along the cable from centre to its attachment. These electric wires are likely to be severely damaged by lighting strokes, especially for the application of de-icing aerial of ground cables of power lines. Moreover, the energy required for mechanical vibrator to keep cable free of ice is not yet known. Nevertheless, both de-icing techniques need a relatively high level of mechanical energy. Indeed, because of the high damping characteristics of strand cables, it is rather difficult to break ice accretions at a point 100 m and more distant from the generator by applying shocks or/and vibrations.
It is an object of the present invention to provide a practical and energy efficient method for de-icing cables.
It is also an object of the present invention to provide a practical and energy efficient apparatus for performing this function.
Accordingly, the present invention provides a method for breaking ice accretions formed on an aerial cable. The aerial cable has a longitudinal axis, a rigidly fixed first end and a second end opposed thereto. The method includes the steps of:
a) gripping a portion of the aerial cable proximate to the second end thereof;
b) rotating the portion of the aerial cable around the longitudinal axis to twist said aerial cable for a predetermined number of turns, thereby accumulating elastic energy therein; and
c) releasing the portion of the aerial cable, thereby liberating the accumulated elastic energy therein, instantly untwisting said cable and breaking the ice accretions formed thereon.
Preferably, steps a) to c) are repeated a number of times sufficient to break and remove all or most of the ice accretions accumulated on the cable.
In accordance with another aspect of the present invention, there is also provided an apparatus for breaking ice accretions formed on an aerial cable. The aerial cable has a longitudinal axis, a rigidly fixed first end and a second end opposed thereto. The apparatus first includes a gripping means for gripping a portion of the aerial cable proximate to the second end thereof. Also provided is a motorised rotating assembly, engageable with the gripping means for rotating the portion of the aerial cable around the longitudinal axis to twist the aerial cable. The apparatus also includes a release mechanism for releasing the portion of the aerial cable. In this manner, the accumulated elastic energy therein is liberated, instantly untwisting the cable and breaking the ice accretions formed thereon.
Advantageously, the present invention preferably uses a twisting elastic force, applied evenly and simultaneously on the whole external surface of the cable, to break and remove all types of ice and snow accretions. It allows the de-icing of a cable with relative simplicity and a very low energy consumption, the de-icing system requiring only the automation of components already in use or in stock, and consuming in average less than 0.05 Watt per meter.
Other features and advantages of the present invention will be better understood upon reading of preferred embodiments thereof with reference to the appended drawings.